In wire communications, generally, distortions of the waveforms occur due to ringing at rising edges and falling edges where data is switched between high level and low level. For example, in a CAN (controller area network) communication of an in-vehicle LAN (local area network), since an output terminal is in a high impedance state during recessive transmission, a large ringing is likely to easily occur. If a signal level is not settled to a level equal to or greater than, or equal to or lower than a threshold voltage for determining the high or low of the bit by a bit determination time point, erroneous determination of the bit is likely to occur. Therefore, arrangement or wiring scales of a communication system such as the number of nodes, the length of wirings, and the number of divergences, and a transmission rate are restricted depending on the occurrence of the ringing.
In order to suppress such ringing, it has been known to detect the switching of the signal level on a transmission line and to reduce the impedance of the transmission line only for a fixed period. Such a technique is, for example, disclosed in JP 2012-257205 A, which corresponds to US 2012/0293230 A1, as a ringing suppression circuit.